In the breeding season during copulation, the female frog shed ova and over these the male shed spermatic fluid. One sperm penetrates the ovum as soon as the male pronucleus.
enter the ovum a second polar body budded off from the ovum .the ovum had the fish polar body below the vitelline membrane.
After this 2nd maturation divides the fusion of the male and female nucleus takes place this is called fertilization ,which is external in case of the frog .
- the cell thus formed is zygote and the nucleus is the zygote nucleus the zygote now divides which is called cleavage or segmentation .
Cleavage and Morula stage
- In the frog, the division is complete but cells formed are equal . So the cleavage is called holoblastic but unequal
the first division is vertical and divides the zygotes into 2 blastomeres. The furrow extended from animal pole to the lower end.the second division is also vertical but at the right angle to the first resulting .
in 4 blastomeres. The division of 3rd is horizontal passing above the equator forming 8 unequal blastomeres.Out of the 4upper, smaller ones are micromeres and lower large are megameres.Two more vertical division take place forming 16 cells and 8 large Abd 8 small cells after thisTwo more vertical division take place forming 16 cells and 8 large Abd 8 small cells after this micromere divide more rapidly than the megameres .this due to lack of yolk in micromers and magameres being filled with
- as the division continues the blastomeres arrange at the periphery and there appears a cavity called blastocoels ,which is filled with fluid thus the embryo appears as the hollow ball and is now called blastula .although blastula appears to be composed of only micro and megameres .The cells forming future parts of body can be identified by special staining methods these areas are :-
1 The animal pole of blastula represents the presumptive ectoderm This can further be presumption epidermis and neural plate .
2 close to notochord lies the presumptive mesoderm.
3 the remainder of the vegetative half formed by large yolk large megameres from the future endoderm.
- this is formed after blastula stage . The conversion of blastula to gastrula is called gastrulation which is completed by 4process:-
- the micromere cells of animal pole divide repeatedly and spread over the lower meamere cells..thus the presumptive notochord,mesoderm, and endoderm get enclosed leaving a small area called yolk plug.
behind the presumptive notochord invagination appears which is the beginning of archenteromns.the open end of this invagination is called blastopore.Its anterior margin is the dorsal lip of the blastopore.As archentron extends inwards it becomes an extensive cavity called archnterons cavity which lies above mega merge cell.
3 Migration of micromere
Micromeres begin to migrate inwards from the dorsal lip and with it, the archenteron enlarges .these from a thick layer or the dorsal surface of archenteron which from the future notochord and mesoderm.when the development of archenteron begins blastopore decreases is the size and gradually disappears .The migration of micromeres also takes place on the side and on the ventral surface of dorsal lip forming lateral lips and ventral lip respectively .these unit and reduce the size of the blastopore . through this comes out yolk filled megameres called yolk plug .
Rotation of the gastrula
The gastrula rotates inside the vitelline membrane . the blastopore comes near the original vegetative pole of the embryo during this yolk plugs moves inwards and moves on the ventral surface of archenteron .After all these changes the following layers can be seen in gastrula .
An outer surface from the ectoderm .
b On the roof of archenteron are the cells which are known as chordamesoderm are not clearly distinguished At the ned of gastrulation a process called neuralation takes place during which notochord formed chordamesoderm.
c floor and side of archenteron form endoderm.
d Still now chordamesoderm and endoderm are not clearly distinguished .At the ned of gastrulation, a process called neurulation takes place during which notochord is formed from chordamesoderm.
- on the mid- dorsal region ectoderm cell thicken to form neural plate. On either side of this is the neural fold.Both neural folds increase in size and fuse at the mid- dorsal region forming the neural canal or tube which opens at the anterior end by the small opening called neuropore and posteriorly remains connected for sometimes with archenteron by the neurentric canal . Now the embryo is called neurula .At the end, the neural tube is covered into a closed tubular canal the anterior part of which is the future brain and the posterior portion forms the spinal cord.
- at the mid- dorsal region the chordamesoderm from a cylindrical rod-like structure which forms the notochord . the rest of chordamesoderm gives rise to mesoderm.on either side of the notochords lies mesoderm which can be divided into three parts :-
1 Dorsally situated epimere,
2 Middle mesoderm called mesomere or nephrotome,
3 Ventral mesoderm called hypomere or lateral plates.
an Epimere furthure has following pasrts
1 Myotomes forming body musculature ,
2 Dermatomes forming dermis of skin and ,
3 Sclerotomes forming the axial skeleton,
b Nephrotome gives rise to excretory and genital organs,
c The lateral plate on either side divide into from 2 layers with a narrow space in between the layers called coelom,
d The outer layer of mesoderm from somatic and inner layer from visceral layer of the coelom.
- the cells forming the floor of archenteron divide , extend dorsally and compeletley enclose archenteron . this layer is below the mesoderm and from the endoderm.
Now the embryo which has elongated to some extent can be said to have three primary germinal layers namely ectoderm on the other side endoderm on the inner side and mesoderm in between them.
Ectoderm : it gives area to epidermis and cutaneous glands and lining of cloaca and mouth cavity along central nerve system – brain and spinal cord and lens cornea ,retina of eye, Olfactory and auditory organs.
endoderm :it forms Epithelium lining of digestive canal except for mouth and cloaca and digestive glands like liver and pancreas . Larynx , trachea, and lungs of respiratory system . Lining of U.B. Thymus and thyroid glands
mesoderm :it is formed dermis of skin, cartilage, and bones of the skeletal system,blood vascular system including blood and vessel , Excretory , genital , spleen, sclerotic and choroid of the eye.
pre- tadpole stage
After 3- 4 days of fertilization the embryo becomes about 4mm in length and lies within the ee=ggs membrane . The body can be distinguished as head ,trunk, and tail. On the head lies on each side one round elevation showing the position of a future ear. On the ventral side of the anterior end is the U-shaped sucker formed by mucous gland cells.Between sucker and the nasal pit is small depression forming stomodaeum. At the posterior end is another depression called protoderm.Posterior to proctodaeum the body elongates to form the tail . internally the embryo contains parts of C.N.S. notochord closed alimentary canal ,liver,heart, and rudiment of the urinary bladder .The embryo is now becoming about 5mm in length and comes out by breaking the eggs membrane .It gets attached by sucker to some aquatic plant.soon from either side of head appears two blunt projection .Then 3rd one is formed and all these three forms branched external gills on either side of the head . This gill and skin helping respiration of the embryo . About 7 days after the embryo emerges from egg membrane the mouth is formed bounded by 2 horny jaws.It now leads a free larvae life and is called tadpole larvae.
it resembles a fish in appearance and feeds one aquatic plants .Its undergoes respiration by external gills.Stomodaeum and proctodaeum get connected with gut forming a complete alimentary canal . its is at fist small and abroad but now becomes long and coiled like a spring.The nutritive substance is present in lower percentage in vegetable food than fresh food.Due to this reason the herbivorous animals have long alimentary canal so that it can absorb more nutritive substance.Slowly 3 pairs of external ills are replaced by 4 pairs of internal gills . Gills covering is called operculum.Mesonephric kidney of adult beings tp appear .Fore and hind limbs start forming Fore limbs take more time than hind limbs as these remain excludes by the operculum.Lungs beigin to appear and when fully formed ,spiracle is closed and external gills disappear.
It is the drastic change which occurs in tadpole to change in an adult .It is under the control of thyroxine which contains iodine.During metamorphic tail is reabsorbed inside the body .Additional iodine in water increases the rate of metamorphosis.
Agrawal, sarita. principle of biology. 2nd edition . kathmandu: Asmita book Publication, 2068 ,2069
Mehta, Krishna Ram. Principle of biology. 2nd edition. kathmandu: Asmita, 2068,2069.
Jorden, S.L. principle of biology. 2nd edition . Kathmandu: Asmita book Publication, 2068.2069.